Plastic net structures are used as reinforcing grids to stabilize concrete and soil in civil engineering and landfill applications. These plastic net structures generally are either uniaxially or biaxially stretched to provide a highly stable, corrosion resistant construction used for such applications as soil stabilization, veneer stabilization, drainage nets in landfills, and concrete stabilization in roads, bridges, and similar structures. Biaxially oriented polypropylene nets have been used to reinforce road beds. Typically plastic net structures are formed of a polypropylene homopolymer or a copolymer of propylene with ethylene or butene.
In applications where the reduction or elimination of creep is important, such as in the stabilization of roads and soil, the reinforcing material should have minimal creep, so that it does not stretch excessively under load. Polyolefin plastic nets are particularly suitable for these applications since they are not subject to hydrolysis. In order to have minimal creep, the plastic web should have a high modulus and be of sufficient thickness so that it deforms to a minimal extent (i.e. exhibits low creep) when a loads are applied to it. Although, presently available plastic reinforcing net structures have proven generally satisfactory for their intended purpose, improved processes and reduced cost structures are desirable.
In the prior art used to produce a biaxially oriented polypropylene netting, the material located at the periphery of the junctions of the machine-direction and cross-machine-direction strands, herein referred to as “nodes”, mainly contains a random molecular orientation. These node regions therefore have undesirably low strength. Moreover the central regions of these nodes tend to be in the form of thick, unoriented humps. These humps constitute areas of weakness, and areas in which the material is inefficiently used. However, the junctions of the machine-direction and cross-machine-direction strands must be strong since these junctions bear a considerable amount of the load when the netting is used for its intended function.
Crystalline polypropylene (also known as “isotactic polypropylene”) is capable of crystallizing in three polymorphic forms: the alpha, beta and gamma forms. In melt-crystallized material the predominant polymorph is the alpha or monoclinic form. The beta or pseudohexagonal form generally occurs at levels of only a few percent, unless certain heterogeneous nuclei are present or the crystallization has occurred in a temperature gradient or in the presence of shearing forces. The gamma or triclinic form is typically only observed in low-molecular weight or stereoblock fractions that have been crystallized at elevated pressures.
The alpha form also is also referred to as “alpha-spherulites” and “alpha-crystals”. The beta form is also referred to as “beta-spherulites”, “beta-crystals”, “beta-form spherulites”, or “beta-crystallinity”. Beta-crystals have a melting point that is generally 10-15° C. lower than that of alpha-crystals.
Generally, extruded polypropylene sheets primarily contain alpha spherulites. Beta nucleants can be added to a polypropylene resin to increase the amount of beta spherulites in the resulting polypropylene sheet.
Porous polypropylene films containing beta spherulites have been used as microporous films. (see U.S. Pat. No. 4,975,469 to P. Jacoby and C. Bauer) The presence of beta nucleants results in the formation of beta spherulites in the sheet, which produce a microporous structure in the resulting stretched film. The micropores allow gases to permeate through the film.
Similarly, beta nucleants have been added to thermoformable thermoplastic resin polypropylene in order to broaden the temperature range over which the sheets can be processed and to prevent sag in the thermoforming oven. (see U.S. Pat. No. 5,310,584 to Jacoby et al.) The beta nucleants induce microvoiding in the sheet when it is deformed during the thermoforming process. Sheets containing high levels of beta spherulites can be thermoformed at lower temperatures than polypropylene sheets formed from alpha nucleated polypropylene or non-nucleated polypropylene since the beta spherulites melt at a lower temperature than the alpha spherulites. This allows the sheets to soften without excessive sag in the thermoforming oven.
Poly propylene net structures must be strong and flexible. They are formed by a process that involves stretching in one or two directions. Thus, sagging in an oven is not a problem in the formation of oriented net structures. Also, if beta spherulites were added, induced microvoiding could lead to an undesirable strength reduction in the oriented strands that comprise the net.
Therefore it is an object of the invention to provide a biaxially oriented polypropylene net that has improved properties and costs less than standard polypropylene nets.
It is a further object to provide a more efficient and less expensive process for making polypropylene nets.